Polymers such as plastics are often processed into forms or finished goods by means of an extruder, such as a screw extruder. The flow rate of extruded material or extrudate is established during continuous feed operation by setting the speed of the screw for the desired product flow rate. The temperature of the extrudate is also an important parameter, as it can affect material properties of the formed product, for example strength and percentage of defects in the product, to name but a few. Accordingly, it is important to control not just the product flow rate but also the extrudate temperature and temperature distribution.
U.S. Pat. No. 4,249,877 describes an extruder that employs a feed screw connected to an annular heated section. External band heaters provide the heat to the heated section to heat the extrudate. A disadvantage with this device is non-uniform heating of the extrudate.
U.S. Pat. No. 3,563,514 describes a plasticizer that employs two rotors having opposed cone-shaped tips which define a working zone therebetween. A disadvantage is that, despite the presence of a small annular region in the barrel positioned between the working zone and the product outlet, no post-working zone heating zone capable of uniformly heating the extrudate is provided.
U.S. Pat. No. 3,070,836 describes an extruder having independent drive means to drive separate screws within a single barrel. Problems with this extruder are that the material flow rate is affected by both drive means and that means for uniformly heating the extrudate is not provided.
An object of this invention is to provide apparatus for forming a heated, melted extrudate having a uniform temperature distribution to thereby produce a plastic having consistent material properties and few defects.